You are right
If you have multiple attacks you must take note of whether these attacks are primary or secondary
A full quote from the SRD:
Most creatures possess one or more natural attacks (attacks made without a weapon). These attacks fall into one of two categories, primary and secondary attacks. Primary attacks are made using the creature’s full base attack bonus and add the creature’s full Strength bonus on damage rolls. Secondary attacks are made using the creature’s base attack bonus –5 and add only 1/2 the creature’s Strength bonus on damage rolls. If a creature has only one natural attack, it is always made using the creature’s full base attack bonus and adds 1-1/2 times the creature’s Strength bonus on damage rolls. This increase does not apply if the creature has multiple attacks but only takes one. If a creature has only one type of attack, but has multiple attacks per round, that attack is treated as a primary attack, regardless of its type.
In your case both of them are primary so you would use your full Attack Bonus and full Strength modifier for damage. You can no longer apply 1.5 Strength modifier bonus to damage, as described quite clearly in the rules.
The intention of that exception is to provide additional power to natural attacks of monsters who one have one in total, regardless of whether this single attack is primary or secondary in the first place.